Leptin, the protein product of the ob gene, is an important circulating signal for regulating body weight, food intake, and energy metabolism in mammals (Zhang Y et al., Nature, 372: 425-432, 1994). These actions are elicited through the binding of leptin to a high affinity receptor in the hypothalamus (Tartaglia L A et al., Cell, 83: 1263-1271, 1995). Although leptin was initially described as a satiety factor that regulates the size of adipose tissue, leptin has many other diverse biological functions. These functions are elicited by the binding of leptin to receptor proteins that are expressed in numerous tissues.
Leptin has been shown to correct the sterility defect in leptin-deficient mice and accelerate puberty when administered to normal mice (Chehab et al. Nature Genet., 12:318-320, 1996). Humans defective in either leptin or the leptin receptor are sterile and sexually immature, supporting leptin's role in reproduction (Stroebel et al., Nature Genet., 18: 213-215, 1998; Clement et al., Nature, 392: 398-401, 1998). Other roles for leptin include a regulator of hematopoeisis (Cioffi et al., Nature Medicine, 2: 585-589, 1996; Gainsford et al., Proc. Natl. Acad. Sci. USA, 93: 14564-14568, 1996), angiogenesis (Bouloumie et al., Circ. Res., 83: 1059-1066, 1998; Sierra-Honigmann et al., Science, 281: 1683-1686, 1998), glucose metabolism (Kamohara et al., Nature, 389: 374-377, 1997), and proinflammatory immune responses (Loffreda et al., FASEB J, 12:57-65, 1998; Lord et al., Nature, 394: 897-901, 1998). Since malnutrition is the leading cause of diminished immunity and increased susceptibility to infection, leptin therapy may also augment the immune response in compromised individuals (Flier, Nature Medicine 4: 1124-1125, 1998).
Although first thought to be produced exclusively by the adipocyte (fat cell), it is now known that leptin is produced in the placenta (Hassink et al., Pediatrics, 100:e1-e6, 1997; Masuzaki et al., Nature Med. 3: 1029-1033, 1997), gastric epithelium (Bado et al., Nature, 394:790-793, 1998), and the mammary gland, as more fully described below (Smith-Kirwin et al., J. Clin. Endocrinol. Metab., 83: 1810-1813, 1998, herein incorporated by reference). The function of placental leptin seems to be as a regulator of fetal growth. Prematurity accounts for a large proportion of infant morbidity, primarily due to respiratory distress, immaturity of organ systems, and poor nutrition. As more fully described below, the premature infant is leptin-deficient, due to early separation from the placenta at a time when they have inadequate adipose tissue. A method to administer leptin to premature and/or poorly growing infants is highly desirable.
Previous leptin therapies rely on the use of recombinant leptin. The administration of recombinant leptin is performed intravenously, intramuscularly, intraperitoneal, and through other parenteral routes to treat obesity, diabetes, and reproductive abnormalities. The amount of leptin that can be delivered by these means is limited by recombinant leptin's poor solubility and by local reactions in skin that occur in response to high doses (Friedman and Halaas, Nature, 395: 763-770, 1998). In rodent models, it has been shown that the ability to optimize the means of administration of leptin may greatly influence its therapeutic effectiveness. Leptin has been shown to be more potent when administered as a subcutaneous infusion than when administered by daily intraperitoneal injections (Halaas et al., Proc. Natl. Acad. Sci. USA, 94: 8878-8883, 1997). Direct administration of leptin into the cerebrospinal fluid (Halaas et al., Proc. Natl. Acad. Sci. USA, 94: 8878-8883, 1997) and by gene therapy means (Chen et al., Proc. Natl. Acad. Sci. USA, 93: 14795-14799, 1996; Murphy et al., Proc. Natl. Acad. Sci. USA, 94: 13921-13926, 1997) have also been shown to be effective for weight loss in rodents, but for safety and ethical concerns have not been tried in humans.
Previous methods involving leptin therapies rely on the use of recombinant leptin. Purification of recombinant leptin involves denaturation of the protein and subsequent renaturation steps. The renatured leptin must then be solubilized, which is problematic because the recombinant leptin aggregates at high concentrations and much protein loss occurs (Guisez et al., Protein Expression Purification, 12: 249-258, 1998). The renatured leptin must then be tested for bioactivity.
Therefore, it is desirable to provide a method for administering leptin that does not require the use or purification of recombinant leptin that can be administered orally, intraveneously, subcutaneously, intramuscularly, intraperitoneally or by other parenteral means. As stated above, a method to administer leptin to premature and/or poorly growing infants is highly desirable.
As more fully described below, the present invention overcomes the problems associated with previous forms of leptin therapy and includes a novel method and composition for administering native leptin that can be administered to subjects who have deficient leptin levels or require leptin therapy to treat a disorder.